Historically, when it was necessary to completely remove a disc from between adjacent vertebrae, the conventional procedure was to fuse the adjacent vertebrae together. This “spinal fusion” procedure, which is still in use today, is a widely accepted surgical treatment for symptomatic lumbar and cervical degenerative disc disease.
More recently, there have been important developments in the field of disc replacement, namely disc arthroplasty, which involves the insertion of an artificial intervertebral disc implant into the intervertebral space between adjacent vertebrae. Such a disc implant allows limited universal movement of the adjacent vertebrae with respect to each other. The aim of total disc replacement is to remove pain generation (caused by a degenerated disc), restore anatomy (disc height), and maintain mobility in the functional spinal unit so that the spine remains in an adapted sagittal balance. Sagittal balance is defined as the equilibrium of the trunk with the legs and pelvis to maintain harmonious sagittal curves and thus the damping effect of the spine. In contrast with fusion techniques, total disc replacement preserves mobility in the motion segment and mimics physiologic conditions.
One such intervertebral implant includes an upper part that can communicate with an adjacent vertebrae, a lower part that can communicate with an adjacent vertebrae, and an insert located between these two parts. To provide an anchor to the adjacent vertebrae, each part includes a vertically extending keel. Examples of this type of implant are disclosed in U.S. Pat. No. 5,314,477 (Marnay) and U.S. Pat. No. 7,204,852 (Marnay et al.), which are hereby incorporated by reference.
While this and other known implants represent improvements in the art of artificial intervertebral implants, there exists a continuing need for improvements for these types of implants.
It will also be noted that in order to provide a keel slot in a vertebra, a cutting of the bone needs to be performed. Typically the cut is made by chiseling, drilling or milling. Combinations of these procedures are possible too. However, where a chisel cut is made using a chisel and a mallet, quite high forces are applied in direction of the cut. With drilling, lesser forces are applied, but the drill can slip of or bend during drilling. With milling, a precise cut is made without high forces, but the milling tool needs to have a certain diameter, because otherwise it will brake during milling so milling is not always possible where a long narrow cut is required. Thus, a procedure used to perform narrow cuts without applying high forces is desirable. Exemplary of such prior art devices and methods are those disclosed in USPA 2004-0215198 (Marnay et al.) and USPA 2006-0064100 Bertagnoli et al.), which are hereby incorporated by reference.